Deep water wells are increasingly being used to extract hydrocarbons. Such deep water wells were previously not considered economical. However, the lack of readily available and easily accessible fields has encouraged significant developments in the extraction of hydrocarbons using deep water wells. However, such deep water wells still have many problems and disadvantages compared to shallow water wells.
In conventional oil and gas wells, it is conventional to have a number of concentric tubes or casings. The outermost casing is secured and fixed in the ground and, in particular, it is fixed within the sea bed. The concentric inner casings are then each secured within the outer casing by being secured to the next adjacent outer casing. Typically, a casing includes a hanger at an upper end thereof. The hanger includes an external shoulder collar which sits on and engages with an internally projecting shoulder the outer casing. Accordingly, the inner casing is effectively supported on and “hung” from the outer casing. Once positioned on the shoulder, cement may be supplied to the annular space defined between the outer surface of the inner casing and the inner surface of the outer casing. This thereby bonds the inner casing to the outer casing. The outer casing may have a return valve operable by a Remote Operated Vehicle located at or adjacent to the mudline. As the cement is pumped down into the annular spacing the excess cement can pass out through valve.
A typical well will include several concentric casings. For example, the outer casing may be cemented to a first inner casing which may support a second inner casing which may support a third inner casing etc. It will be appreciated that it is relatively easy for the excess cement between the outer casing and the first inner casing to be easily extracted out of the well through a valve located at the mudline in the outer casing. However, it becomes increasingly difficult to simply extract the excess cement from between successive inner casings whilst maintaining the integrity of the subsea wellhead.
In addition, it is preferable to have the inner concentric casings locked down such that the casing is not lifted upwards by any excess pressure or force produced in the annular space surrounding it. Such lockdown connectors may require the hanger to have a locking arrangement which can be relatively difficult to operate and manipulate since the lockdown connectors are located a long distance from the surface. Furthermore, such lockdown arrangements may be complex and may not provide any axial loading on the casing string.
Prior art systems may include multiple components including annular sealing components for creating the required seal, locking components for locking a well casing string against downwards movement and also locking components for locking the well casing string against upwards movement. Each of these components requires activation or actuation which may only occur whilst they are located at a deep sea level. Accordingly, these multiple components and the activations can be difficult and problematic.
It is an aim of the present invention to overcome at least one problem associated with the prior art whether referred to herein or otherwise.